This invention relates to a plane positioning apparatus and, more particularly, to one suitable for application to an autofocusing mechanism or an autoleveling mechanism in a so-called step-and-scanning type exposure apparatus which sequentially exposes a pattern on a reticle onto each shot area on a photosensitive substrate by synchronously scanning the reticle and the substrate in respect to a slit radiating region that is, for example, a rectangular or an arc in shape.
Conventionally, in manufacturing a semiconductor device, a liquid crystal display element, or a thin film magnetic disk using a photolithography technique, a projection exposure apparatus has been used for exposing a pattern on a photomask or reticle (hereinafter generally called a xe2x80x9creticlexe2x80x9d) onto a wafer (or glass plate, etc.) on which photoresist or the like is applied, through an optical projection system. Generally, because high resolution is required for the projection exposure apparatus, and the numerical aperture of the installed optical projection system is high, the depth of focus (focus margin) of the projected image is reduced in inverse proportion to the square of the numerical aperture. Then, to align each shot region of a wafer within the range of the depth of focus in respect to the imaging plane of the optical projection system, the projection exposure apparatus has been provided with an autofocus mechanism which aligns the focus position of the wafer, at a predetermined reference point in an exposure field, with the imaging plane by the optical projection system, and an autoleveling mechanism which sets the inclination of the exposure surface of the wafer in the exposure field parallel to the imaging plane.
A conventional autofocus mechanism, on the one hand, comprises a focus position detection sensor for detecting the amount of defocus in the focus position (the position of the optical projection system in the direction of the optical axis) of a predetermined measurement point in each shot region of the wafer from the imaging plane (hereinafter called the AF sensor), and a servo system for controlling the height of the Z stage to make its amount of defocus fall within an allowable range. In the AF sensor, a grazing-incidence detector reimages the slit pattern image, which is projected on a predetermined measurement point in the exposure field at an angle, on the reception section, and detects the focus position at the measurement point by utilizing the fact that the deviation of the focus position on the wafer surface causes the position of the reimaged slit pattern image to change.
On the other hand, the autoleveling mechanism comprises a leveling sensor which detects focus positions at three or more measurement points on each shot region on the wafer, and a servo system which makes the mount of shift of the inclination of the average plane determined by the focus positions at these three or more measurement points within an allowable range.
In this regards, in a conventional batch projection exposure apparatus, such as the stepper which is generally used, because the wafer whose focus position is subject to detection is stationary during exposure, it is possible to manage the decrease in the depth of focus by improving the resolution and accuracy of the AF sensor and the leveling sensor which detect the mount of defocus, and by tuning the accuracy of the mechanism for the Z stage in the servo system, even if the numerical aperture of the optical projection system is further increased.
Recently, an one chip pattern for a semiconductor and the like has come to have a larger and larger size so that the projection exposure is required to have a larger size for projecting a pattern with a larger area on the reticle onto the wafer.
It has also become necessary to improve the resolution of the optical projection system as the pattern for a semiconductor and the like becomes finer. There is a disadvantage in that it is difficult to enlarge the exposure field of the optical projection system in its design or manufacture to improve the resolution of the optical projection system. In particular, when a catadioptric system is used as the optical projection system, an exposure field without aberration is given by a circular arc in shape.
To manage such an enlargement of the transfer pattern and the constraints of the exposure field of the optical projection system, a so-called step-and-scanning type projection exposure apparatus has been developed which synchronously scans a reticle or wafer to, for example, a rectangular, circular arc, or a hexagonal radiating region (hereinafter called a slit radiating region) to sequentially project a pattern on the reticle, which has a larger area than the slit radiating region, onto each shot region on the wafer.
Such a projection exposure apparatus also requires an autofocus mechanism and an autoleveling mechanism, both of which align the exposure surface of the wafer under exposure with the imaging plane. However, in the step-and-scanning type system, because the wafer whose focus position is to be detected moves during exposure, the output signal of the AF sensor or leveling sensor indicating the focus position of the measurement point varies as a function of the position in the scanning direction. Thus, if signal processing and control similar to that of the batch projection exposure apparatus is conducted, it results in poor follow-up capability in respect to the movement of the focus position of the wafer. Therefore, there is the disadvantage such that the exposure surface of the wafer is difficult to align with the imaging plane within the depth of focus. This disadvantage is described in detail in the following:
In the step-and-scanning type projection exposure apparatus, as described-earlier, the detection signal of a focus position is observed as a function of the position in the scanning direction in a time series. Therefore, when this signal is merely caused as a deviation signal to operate a closed-loop servo, control is performed in such a manner that the Z stage dynamically follows up the time-series signal if the system has a sufficiently fast response. If the width of the slit exposure region (exposed field) in the scanning direction is sufficiently small in respect to the scanning speed, this causes no particular disadvantage. However, the width of the slit exposure region in the scanning direction usually has a non-negligible value in respect to the scanning speed.
Therefore, even if the center of an exposure region is moved while caused to completely follow the detection signal of a focus position, the movement of the Z stage when a point on the wafer passes through the slit exposure region adversely affects the imaging characteristics at that point as vibration. Moreover, because the slit exposure region has a finite width in the scanning direction, follow-up essentially cannot be performed for unevenness finer than the limit, which is the unevenness of one cycle in the width on the wafer. The best control in this case is to perform no control. If the center of the exposure region is caused to follow the fine unevenness as in the above, any control may deteriorate an image by deterioration of the focusing accuracy.
On the one hand, the wider the width of the slit exposure region along the scanning direction, the higher the step-and-scanning type system throughput. On the other hand, if this width is large, light may leak from an area outside the blocking area at the end of the reticle to cause unnecessary exposure. Thus, as disclosed in Japanese Patent Public Disclosure No. HEI 4-196513, a method has been developed wherein the exposure region is made narrower in the scanning direction near the starting and terminating ends of the reticle, while it is made wider in the scanning direction between the starting and terminating ends of the reticle. Such a method wherein the width of the exposure region in the scanning direction is changed during exposure is not attained in a conventional stepper or the like, so a preferable control method for focusing and leveling meeting the change in the shape of the exposure region has not been devised.
In view of the above, this invention is intended to provide a plane positioning apparatus for a step-and-scanning type exposure apparatus which can hold the exposure surface of the wafer, and balance between holding of the exposure surface of the wafer within the depth of focus and the prevention of image degradation to enable optimum autofocus and autoleveling control by controlling the follow-up capability to the focus position of the exposure surface of a moving wafer in a suitable state. More particularly, it is intended to provide a plane positioning apparatus which can provide optimum autofocus and autoleveling control even when the shape of the exposure range changes during the exposure operation.
To attain the above objects, the first plane positioning apparatus according to the present invention is a plane positioning apparatus installed on a scanning exposure apparatus, which comprises a mask stage for scanning a mask formed with a transfer pattern in a predetermined direction in respect to a radiating region with a predetermined variable shape, and a substrate stage for scanning the photosensitive substrate in a predetermined direction in synchronization with the mask stage, and which sequentially exposes the pattern of the mask onto the substrate, the plane positioning apparatus being for aligning the exposure plane of the substrate with a predetermined reference plane, and comprising:
a plane positioning means installed on the substrate stage for aligning a predetermined approximate plane of the exposure plane of the substrate with the predetermined reference plane;
a height detection means for detecting the height of the exposure plane of the substrate at a plurality of measurement points in a measurement region before the exposure region of the pattern of said mask with respect to the scanning direction of the substrate; and
an approximate plane calculation means for finding an approximate plane of the exposure plane of the substrate-by using a plurality of height information on the exposure plane of the substrate from the height of the plurality of measurement points detected by the height detection means, which information is measured in the variable exposure region of the pattern of the mask which varies according to the variable radiating region;
the approximate plane found by the approximate plane calculation means being aligned with the predetermined reference plane by the plane positioning means.
In addition, in the first plane positioning apparatus, the approximate plane calculation means includes a filter means which has low-pass characteristics filtering the shape of a plane, which is formed by arranging the height of the plurality of measurement points detected by the height detection means, on a spatial frequency range, an approximate plane of the exposure plane of the substrate being found from information after filtering a plurality of height information on the exposure plane of the substrate measured in the variable exposure region of the pattern of the mask with the filter means;
cutoff spatial frequencies in the amplitude transmission characteristic of the filter means in the scanning direction of the substrate and in a nonscanning direction normal to the scanning direction of the substrate being set proportional to the inverse numbers of the width of the exposure region of the pattern of the mask in the scanning direction and the width in the nonscanning direction;
the approximate plane found by the approximate plane calculation means being aligned with the predetermined reference plane by the plane positioning means.
Furthermore, in the first plane positioning apparatus, it may be possible to provide an optical projection system with a projection magnification xcex2 between the mask and the substrate, the mask stage scanning at a speed of VR in a predetermined direction in a plane normal to the axis of the optical projection system, the substrate stage scanning at a speed of xcex2xc2x7VR in a direction opposite to the predetermined direction in a plane normal to the axis.
Another aspect of the first plane positioning apparatus is a plane positioning apparatus installed on a scanning exposure apparatus, which comprises a mask stage for scanning a mask formed with a transfer pattern in a predetermined direction in respect to a radiating region with a predetermined variable shape, and a substrate stage for scanning a photosensitive substrate in a predetermined direction in synchronization with the mask stage, and sequentially exposes the pattern of the mask onto the substrate, the plane positioning apparatus being for aligning the exposure plane of the substrate with a predetermined reference plane, and comprising:
a drive device for the substrate stage installed on the substrate stage and being capable of aligning a predetermined approximate plane of the exposure plane of the substrate with the predetermined reference plane at the same level and in parallel to the reference plane;
a sensor for detecting the height of the exposure plane of the substrate at a plurality of measurement points in a measurement region before the exposure region of the pattern of the mask with respect to the scanning direction of the substrate; and
a computer for finding the predetermined approximate plane of the exposure plane of the substrate by using a plurality of height information on the exposure plane of the substrate from the height of the plurality of measurement points in the measurement region, which information is measured in the variable exposure region of the pattern of the mask which varies according to the variable radiating region.
In still another aspect of the first plane positioning apparatus, a scanning exposure apparatus comprises:
a illuminating system for radiating illuminating light on a mask through an opening of a variable field diaphragm positioned on a plane substantially conjugate with the pattern plane of the mask;
an optical projection system for projecting a pattern formed on the mask onto a photosensitive substrate;
a movable member for moving, during scanning exposure, the mask and the substrate in a direction substantially normal to the axis of the optical projection system;
a drive member for changing the opening width of the variable field diaphragm in interlocking with the movement of the mask;
a sensor having a plurality of measurement points in a region before the exposure region of the pattern of the mask with respect to the direction of movement of the substrate, and detecting the position of the surface of the substrate in a direction of an optical axis of the optical projection system at each of the plurality of measurement points;
a calculator for, during the scanning exposure, calculating an approximate plane of the surface of the substrate based on multiple positions in the exposure region of the pattern of the mask varying according to the opening of the variable field diaphragm of multiple positions-detected by the sensor; and
a device for moving the substrate and an imaging plane of the optical projection system relative to each other so as to substantially match the approximate plane calculated with the imaging plane.
In addition, according to the first plane positioning apparatus, the calculator includes a filter member which has low-pass characteristics filtering the shape of a plane, which is formed by arranging the height of the plurality of measurement points detected by the sensor, an approximate plane of the exposure plane of the substrate being found from information on the exposure plane of the substrate measured in the variable exposure region of the pattern of the mask with the filter member.
A second plane positioning apparatus according to the present invention to attain the above objects is a plane positioning apparatus installed on a scanning exposure apparatus, which comprises a mask stage for scanning a mask formed with a transfer pattern in a predetermined direction in respect to a radiating region with a predetermined shape, and a substrate stage for scanning a photosensitive substrate in a predetermined direction in synchronization with the mask stage, and which sequentially exposes the pattern of the mask onto the substrate, the plane positioning apparatus being for aligning the exposure plane of the substrate with a predetermined reference plane, and comprising:
a plane positioning means installed on the substrate stage for aligning a predetermined approximate plane of the exposure plane of the substrate with the predetermined reference plane;
a height detection means for detecting the height of the exposure plane of the substrate at a plurality of measurement points in a measurement region near the exposure region of the pattern of said mask; and
a filter means having low-pass characteristics filtering the shape of a plane, which is formed by arranging the height of the plurality of measurement points detected by the height detection means, on a spatial frequency range;
an approximate plane calculation means for finding an approximate plane of the exposure plane of the substrate in the exposure region of the pattern of said mask from the shape of the plane after filtering by the filter means;
cutoff spatial frequencies in the amplitude transmission characteristic of the filter means in the scanning direction of the substrate and in a direction normal to the scanning direction of the substrate being set proportional to the inverse numbers of the width of the exposure region the pattern of the mask in the scanning direction and the width in the nonscanning direction;
the approximate plane found by the approximate plane calculation means being aligned with the predetermined reference plane by the plane positioning means.
In addition, according to the second plane positioning apparatus, the shape of a region through which spatial frequency components in the amplitude transmission characteristic of the filter means pass can be made similar to the shape of the exposure region of the pattern of the mask.